1. Field of the Invention
The present invention relates to an image coding apparatus and, more particularly, to an image coding apparatus for coding an image signal by adaptively switching coding based on an inter-mode (interframe (or interfield) prediction coding) and coding based on an intra-mode (intraframe (or intrafield) coding).
2. Related Background Art
Various coding schemes have been proposed to reduce the data quantity in coding a motion image signal.
According to one of the proposed coding schemes, coding is performed by switching coding based on the intra-mode (intraframe coding) and coding based on the inter-mode (interframe prediction coding).
A motion image can be regarded as a continuous display of independent images along a time axis.
Intraframe coding is a compression coding scheme suitable for an image having a low correlation in the direction of time, e.g., an image exhibiting quick movement, or an image having no correlation in the direction of time, e.g., an image having undergone a scene change. That is, intraframe coding is designed to perform compression coding by using similarities between adjacent pixels.
Interframe prediction coding is a compression coding scheme suitable for an image exhibiting a small movement. That is, interframe prediction coding is based on the fact that the correlation between frames of an image exhibiting a small movement is high in the direction of time. In interframe prediction coding, therefore, compression coding is performed by calculating prediction values of image data to be coded from an adjacent image, calculating differences between the calculated prediction values and the image data to be coded, and coding the calculated differences.
It is known that in a conventional coding apparatus, when an error occurs in data in coding based on the inter-mode, the error generally propagates.
Coding based on the intra-mode is, therefore, forcibly performed at predetermined time intervals (e.g., one sec) to eliminate the influences of errors mixed in data at the predetermined time intervals. That is, refresh processing is performed.
Assume that coding based on the intra-mode is periodically performed for refresh processing, and for example, refresh processing is performed for a high-resolution image or an image exhibiting a low correlation between frames. In this case, since the quantization step for a reduction in data quantity in coding is abruptly coarsened, a deterioration in image quality become conspicuous. In addition, if images like the above images continuously appear, a deterioration in image quality caused by refresh processing occurs at every refresh period, making the deterioration in image quality even more conspicuous.